1. Field of the Invention
The present invention relates to a method of manufacturing pneumatic tires, and more specifically to a method of manufacturing pneumatic radial tires of excellent speed durability in accordance with a simple and efficient manufacturing process.
2. Description of the prior Art
In pneumatic radial tires used for automotive vehicles, trucks, buses, etc., there exists no special difference in the tread contour and the belt structure between ordinary tires and ultra-high speed tires. For instance, the tread is formed with a plurality of circumferential grooves and lateral grooves so as to form ribs and/or blocks.
Further, the contour of the tread formed with ribs and/or blocks is usually formed into a circular arc shape, when seen in a cross-sectional view taken along a surface extending along a radial direction of a tire. Furthermore, the shape of the inextensible belt layer reinforced by metallic cords is also usually formed in the same shape substantially in parallel to the tire tread contour.
Conventionally, the pneumatic radial tires as described above are manufactured in accordance with process as shown in FIGS. 1(A), (B) and (C).
(1) With reference to FIG. 1(A), a carcass ply 2 is wound around a cylindrical former 1. Here, the carcass 2 is composed of one, two or three (at the most) rubber coated plies formed by arranging fiber cords (e.g. made of polyester, polyamide, rayon, aromatic polyamide, etc.) in a direction substantially in parallel to the rotary shaft of the former 1 (perpendicular to the tire equatorial plane).
Thereafter, two bead rings 3 are set to both sides of the former 1 coaxially with the former 1; two bead filler rubbers 4 are arranged on these two bead rings 3; both the ends of the carcass ply 2 are bent radially outward so as to wrap up the bead cores 3, respectively; and a pair of side rubbers 5 are attached to both the sides of the carcass ply 2, while leaving a space at the center of the carcass ply 2.
(2) With reference to FIG. 1(B), the carcass portion is expanded into a toroidal shape with a bladder press 6; and a belt member 7 and a tread rubber 8 are adhered onto the carcass ply 2 and between both the side rubbers 5.
Here, the belt layer 7 is formed by stacking a plurality of rubber coated layers in such a way that the arranged cords of different layers intersect each other. Each belt layer is formed by arranging metal cords at a small inclination angle with respect to the tire circumferential direction. In this process, the belt layer 7 is first adhered onto the crown portion of the carcass ply 2 and then the tread rubber member 8 is adhered onto the belt layer 7, separately. Alternatively, first the belt layer 7 and the tread rubber member 8 are formed integral with each other, and then the two belt layer 7 and the tread rubber member 8 are adhered together onto the crown portion of the carcass ply 2.
Further, the tread rubber member 8 is composed of a main tread rubber member 8A (trapezoidal shape in cross section) of high abrasion resistance and two miniskirt rubber members 8B made of material similar to the side rubber 5 and adhered to both sides of the tread rubber member 8A. However, it is also possible to form the tread rubber members 8A and 8B integral with each other. In this case, it is preferable that the radially outward ends of the side rubbers 5 are adhered so as to cover both the side ends of the tread member 8.
(3) With reference to FIG. 1(C), a stitching roller 9 is brought into contact with the side rubbers 5 to obtain a green tire 10 having a trapezoidal cross section such that the radially outermost end thereof is substantially flat.
(4) With reference to FIG. 2, a green tire 10 thus formed is put into a vulcanization mold (not shown) formed with ribs for making circumferential grooves and then vulcanized at high temperature and under high pressure as is usual, in order to obtain a pneumatic radial tire T as shown in FIG. 2. As depicted in FIG. 2, the pneumatic tire formed in accordance with the prior-art manufacturing method includes a carcass play 2, belt layers 7 formed on the carcass ply 2, and a tread rubber member 8 on which ribs and/or blocks 8C and circumferential grooves 8D are formed alternately.
In the pneumatic radial tire manufactured in accordance with the prior-art method and inflated under predetermined pressure, the contour of the tread rubber member 8 is formed as shown by solid lines in FIG. 2. No problem may arise when the tire is rotating at low speed or under the normal travel condition. However, when the tire is rotating at such a high speed as 200 to 300 km/h, the tread tends to expand radially outward, in particular the center of the ribs and/or blocks 8c project outward due to centrifugal force generated by the volume thereof as shown by dot-dashed line in FIG. 2.
When the radially outward surface of the tread expands at the center thereof, since the contact pressure against the ground locally increases excessively and further the tensile and compressive stress are repeated at the ribs or blocks, the temperature of the tread rubber member 8A rises and therefore the strength of the rubber drops sharply. That is, there exist a problem in that the ribs and/or blocks 8c of the tread are blow away or peeled off (so-called blown chunk-out). To overcome these problems, although it may be possible to use a rubber of high temperature resistance as the tread rubber member, it has thus far been difficult to obtain a satisfactory effect.
SUMMARY OF THE INVENTION
With these problems in mind, therefore, it is the primary object of the present invention to provide a method of manufacturing a pneumatic tire excellent in high speed durability in spite of a relatively simple method of manufacture.
To achieve the above-mentioned object, a method of manufacturing a pneumatic tire, according to the present invention comprises the following steps of: (a) winding a carcass ply around a cylindrical former; (b) adhering a pair of bead rings and a pair of bead filler rubbers on both side ends of the carcass ply; (c) adhering a pair of side rubbers on the bead filler rubbers, while leaving a space at a central portion of the carcass ply; (d) expanding the carcass ply on which bead rings, bead filler rubbers and side rubbers are arranged with a bladder press into a toroidal shape; (e) adhering a belt layer on the carcass ply; (f) adhering a tread rubber member whose outer surface is formed into a corrugation shape along the tire axial direction, when viewed in cross section, so that radially convex portions thereof may be located under tire circumferential grooves to be formed after vulcanization process and radially concave portion thereof may be located under tire ribs and/or blocks also to be formed after vulcanization process; (g) adhering both side ends of the tread rubber member to the side rubbers, respectively, with a stitching roller to obtain a green tire; and (h) placing the green tire into a mold for vulcanization, whereby radially concave belt layer portions can be formed just under the tire circumferential grooves and radially convex belt layer portions can be formed under the tire ribs and/or blocks before the tire is not yet inflated.
A height "h" of the corrugation on the outer surface of the tread rubber member lies preferably within a range from 0.5 to 3.0 mm between the radially concave and convex portions thereof. Further, the height of the corrugation is preferably the largest under the central rib and/or block and decreases from under the central rib and/or block to under side ribs and/or side blocks along the tire axial direction.